1. Field of the Invention
The invention relates to a control device and a control method for a turbocharger-equipped internal combustion engine, and more particularly to an exhaust valve control during lean combustion.
2. Description of the Related Art
There is a known device (independent exhaust type engine) that includes a first exhaust valve that opens and closes an exhaust passageway that leads to a turbine, and a second exhaust valve that opens and closes an exhaust passageway that does not lead to the turbine (see, e.g., Japanese Patent Application Publication No. 10-89106 (JP-A-10-89106)). According to this device, during a high-speed and high-load operation, the second exhaust valve that does not communicate with the turbine is opened at an earlier timing than the first exhaust valve that communicates with the turbine. This reduces the exhaust pumping loss, so that the output can be improved in a high-speed region.
During the lean combustion, the exhaust gas temperature becomes low, and therefore the work done by turbocharger becomes small, so that the charging pressure may sometimes become insufficient. Besides, because the catalyst bed temperature declines, the exhaust emission characteristic may sometimes deteriorate.